These guys have been having a hard time since the world froze over. They couldn't survive except in the areas near the equator where it was nearly temperate. In order to continue to exist, they took drastic measures.

A large kingdom of them decided that their civilization must outlast this potentially endless winter, and used their resources to create a system of chambers beneath their city in which their children could grow. Then, in a massive, profane ritual; they stripped the life from their bodies and used it to power the furnaces that keep the next generation warm and alive. As each generation grows, they are kept away from the world to remain safe; and they are fed, guarded, and tended to by their undead elders. As the bodies of the elders degenerate further to the point that they lose their function, they feed their bodies to the furnaces as well. At this point the generation before has their life force stripped out to refresh the fallen undead and a new generation is hatched to preserve their people.